1. Field of the Invention
The present invention relates to tanks for liquid oxygen. More particularly, the present invention relates to composite tanks for liquid oxygen.
2. General Background of the Invention
The following U.S. Patents are incorporated herein by reference: U.S. Pat. No. 5,056,367; 5,251,487; 5,380,768; 5,403,537; 5,419,139; and all references cited in those patents.
The following international applications published under the PCT are incorporated herein by reference: International Publication Nos. WO 97/18081 and WO 97/28401 and all references cited in those publications.
A fiber-reinforced composite is defined as a material consisting of fibers of high strength and modulus embedded in or bonded to a matrix with distinct interfaces or boundaries between them. In this form, both fibers and matrix retain their physical and chemical identities, yet they produce a combination of properties that cannot be produced by either constituent alone. In general, fibers are the principal load carrying members, while the surrounding matrix keeps them in desired location and orientation, transfers loads between fibers, and protects the fibers. The matrix material may be a polymer, a metal, or a ceramic. This disclosure focuses on polymer matrix composites.
The fibers can be made from a variety of materials. These materials include glass, graphite or carbon, polymers, boron, ceramics, or metals. Glass fibers include E-glass (electrical) and S-glass (structural) types. Carbon fibers include those made from different precursors, such as polyacrylonitrile (PAN) or pitch. Polymer fibers include, but are not limited to, aramid (Kevlar®), polyethylene (Spectra®), or PBO (Zylon®). Ceramic fibers may include silicon carbide (SiC) or aluminum oxide (Al2O3).
For cryogenic tanks, the preferred matrix material is a polymer. The preferred fiber is carbon fiber, more preferably PAN-based fibers, more preferably high strength (over 500 ksi) and high modulus (over 30 Msi) fibers. The most preferred fibers are ultra high modulus fibers (over 60 Msi), specifically M55J fiber by Toray.
The development of polymer matrix composite liquid oxygen tanks is a critical step in creating the next generation of launch vehicles. Future reusable launch vehicles need to minimize the gross liftoff weight (GLOW) by reducing the dry mass fraction. The (dry) mass fraction is the weight of the launch vehicle without fuel divided by the weight of the vehicle with fuel. FIG. 1 is graph showing the effect of mass fraction on GLOW. Indicated on the graph is the RLV mass fraction target region as well as the mass fraction of the RLV without the weight reduction that composites could provide. It is clear that composite tanks are critical to enable future launch vehicles to meet required mass fractions.
The required mass fraction is possible due to the reduction of weight that composite materials can provide. Traditional oxygen tanks are usually made from metals. The space shuttle external tank (ET) has historically been made from 2219 aluminum and more recently 2195 aluminum/lithium alloy. FIG. 2 shows a comparison between these two aluminum alloys and a typical composite material for a liquid oxygen tank for a launch vehicle. The chart shows that a composite tank provides up to 41% and 28% weight savings when compared to 2219 and 2195 aluminum tanks, respectively.
In addition to meeting the required mass fraction, a liquid oxygen tank must be compatible with oxygen. The ASTM definition for oxygen compatibility is the “ability of a substance to coexist with both oxygen and a potential source(s) of ignition within the acceptable risk parameter of the user.” It is imperative that materials are selected that will resist any type of detrimental, combustible reaction when exposed to usage environments. Typically, non-metallic materials are not used in these applications because most are easily ignited in the presence of oxygen. However, there are some polymeric materials with inert chemistries that may be used for this application and resist ignition. These chemistries were evaluated by fabricating coupons and testing them with various ignition mechanisms in the presence of liquid and gaseous oxygen. The testing performed reflected situations in launch vehicles that could be potential sources of ignition in composite. These tests included pressurized mechanical impact, particle impact, puncture, puncture of damaged, oxygen-soaked samples, electrostatic discharge, friction, and pyrotechnic shock.
Applications include liquid oxygen for future launch vehicles, such as the Lockheed Martin Reusable Launch Vehicle (RLV). They could also potentially be used in other aerospace applications, including but not limited to, RFP (rocket fuel propellant) tanks and crew vehicle cabins. Other industries that may be interested in composite oxygen tanks include the air handling and medical industries. The ability to resist ignition may also be useful in chemical storage tanks and NGV (natural gas vehicle) tanks.